The present invention relates generally to projection systems for pictures and projection display apparatus, and more particularly to a projection system for pictures which is capable of improving the luminance and contrast of the magnified picture projected onto a projection screen and a projection display apparatus therefor.
A conventional projection display apparatus of the sort described above is constructed so that the source picture formed on the face plate panel of a projection cathode-ray tube or a liquid crystal panel is projected onto a projection screen by means of an optical system for magnifying whose magnifications in both horizontal and vertical directions are equal.
In the projection display apparatus using such a projection cathode-ray tube, the face plate panel is bombarded with a electron beam having a great current to form a Source picture and the magnifying projection optical system is used to project the source picture onto the projection screen.
In order to intensify the luminance and contrast of the picture projected onto the screen by strengthening the luminance of the source picture formed on the face plate panel, a current density needs increasing while the spot size of the electron beam for use in forming the source picture is decreased as disclosed in Japanese Patent Laid-Open No. 561/1979. However, the temperature of the face plate panel is caused to rise because of the bombardment of the electron beam at the large current and this results in reducing the light emission efficiency of the fluorescent material and consequently reducing the luminance and contrast of the magnified picture projected onto the projection screen. Moreover, the projection cathode-ray tube and the magnifying projection optical system, when positioned close to each other, may admit of reflection of light therebetween, thus allowing a reduction in the luminance and contrast of the picture on the screen likewise.
Attempts have been made to solve the foregoing problems; in the case of the former problem, for instance, Japanese Patent Laid-Open No. 90628/1973 discloses the provision of a cooling construction for a face plate panel so as to prevent its temperature from rising.
As what was intended to solve the latter problem, moreover, Japanese Patent Laid-Open No. 194234/1983 discloses the provision of a reflection preventive medium between a face plate panel and an optical system.
Further, the Television Society Journal `Projection Large Screen Display,` pp 164-, February 1991, for instance, may be quoted as what discloses a general system configuration relative to a projection display of the sort stated above.
When the prior art projection cathode-ray tube is used to deal with a picture whose vertical length is different from horizontal length, particularly when a picture like a high-vision picture having an extremely high aspect ratio of a horizontal to vertical ratio is referred to as "an aspect ratio" is projected, the source picture generated on the face plate panel of the projection cathode-ray tube becomes oblong.
As the source picture formed on the face plate panel is restricted by the size of its long side (generally the size in the horizontal or lateral direction), an area of the picture within the (effective) size that can be covered by a projection optical system relatively decreases for higher aspect ratio and this results in reducing the luminance and contrast of the picture projected onto a projection screen as the whole luminous flux on the screen abates. This problem arises not only when the projection cathode-ray tube is used as a means for generating the source picture but also when a means for generating a picture such as a liquid crystal panel is employed.